GABA-induced uncoupling of GABA/benzodiazepine site interactions is mediated by increased GABAA receptor internalization and associated with a change in subunit composition

摘要

Persistent activation of GABAA receptors triggers compensatory changes in receptor function that are relevant to physiological, pathological and pharmacological conditions. Chronic treatment of cultured neurons with GABA for 48 h has been shown to produce a down-regulation of receptor number and an uncoupling of GABA/benzodiazepine site interactions with a half-time of 24–25 h. Down-regulation is the result of a transcriptional repression of GABAA receptor subunit genes and depends on activation of L-type voltage-gated calcium channels. The mechanism of this uncoupling is currently unknown. We have previously demonstrated that a single brief exposure of rat primary neocortical cultures to GABA for 5–10 min (t½ = 3 min) initiates a process that results in uncoupling hours later (t½ = 12 h) without a change in receptor number. Uncoupling is contingent upon GABAA receptor activation and independent of voltage-gated calcium influx. This process is accompanied by a selective decrease in subunit mRNA levels. Here, we report that the brief GABA exposure induces a decrease in the percentage of α3-containing receptors, a receptor subtype that exhibits a high degree of coupling between GABA and benzodiazepine binding sites. Initiation of GABA-induced uncoupling is prevented by co-incubation of GABA with high concentrations of sucrose suggesting that it is dependent on a receptor internalization step. Moreover, results from immunocytochemical and biochemical experiments indicate that GABA exposure causes an increase in GABAA receptor endocytosis. Together, these data suggest that the uncoupling mechanism involves an initial increase in receptor internalization followed by activation of a signaling cascade that leads to selective changes in receptor subunit levels. These changes might result in the assembly of receptors with altered subunit compositions that display a lower degree of coupling between GABA and benzodiazepine sites. Uncoupling might represent a homeostatic mechanism that negatively regulates GABAergic transmission under physiological conditions in which synaptic GABAA receptors are transiently activated for several minutes.